In molding an optical lens, when a mold is designed and produced by using designing values of the optical lens for the mold as they are, the optical lens produced with the mold sometimes is not produced in the same shape as that of the designing values. This is because of mold shrinkage being dependent on the material, stress due to the shape of the optical lens and the like, and because the mold surface of the mold is not transferred to the lens surface with high precision.
For example, when molding is performed by using a mold having a spherical molding surface to mold a spherical lens, the molded optical lens sometimes has a surface shape other than a spherical surface including an aspherical shape. Therefore, in designing a mold, it is necessary to add a proper shape correction to the mold in consideration of these various factors.
Correction values of the molding surfaces of these molds differ for each refractive power of optical lenses, lens material and shape of the design curved surface, and have complicated tendency by combination of them. In order to determine a suitable correction value, it is necessary to experimentally verify the actual deformation in each mold.
Further, prediction and quantification of the correction value are difficult, and a skill is required in determination of a proper correction value.
A concrete operation includes: (a) molding all kinds of optical lenses with the corresponding molds by tests, and (b) measuring errors with respect to the designing values of the optical lenses; (c) calculating temporary correction values (empirical values) by multiplying the measured errors by various coefficients and remake the molds; (d) molding optical lenses with the remade molds by tests again, and (e) measuring the shape errors of the optical lenses. It is a general method to repeat the above-described (c) to (e) to optimize correction.
In order to perform the operation of optimizing such shape correction of a mold, however, a number of molding tests are required. Especially in the case of spectacle lenses, various kinds of molds are required. Namely, spectacle lenses, lenses corresponding to prescriptions of the individual spectacle-lenses wearers have to be prepared. For example, when the range of the refractive power at vertex of a spherical diopter is −8.00 diopter (D) to +6.00 diopter (D), and the division unit of the refractive power is 0.25 D pitch, as for the diopter range of the spectacle lenses corresponding to the prescriptions, the number of kinds of spherical diopters is 56.
Further, in the case of the cylindrical refractive power corresponding to an astigmatic prescription being in the range of 0.25 diopter (D) to 2.00 diopter (D), when the division unit of refractive power is 0.25 D pitch, eight kinds are required as the kind of astigmatism. Therefore, when the spherical prescription and astigmatic prescription are combined, it is necessary for a product to prepare for 448 kinds of lens diopters, and since the mold is composed of two molds that are upper and lower molds, the number of kinds of molds becomes 896 in total.
For this reason, in production of molds, the operation of shape correction for each mold as described above is performed, and therefore, a long manufacturing period is required.
Meanwhile, as for a method for making correction which is added to the molding surface of a mold, there is known for a method for correcting a mold by obtaining a spherical shape having a single curvature by using the least squares method so that errors of the molded optical lens and the design values of the optical lens become the minimum, and by using the curvature of the spherical shape as an average curvature (the first prior art).
Further, as the second prior art, there is a method for applying a predicted value as a correction value when deformation in consideration of shrinkage is predictable in the case of a simple shape (Patent Document 1).
Further, as the third prior art, there is a method for making correction based on a shape error measurement value which is obtained by measuring a three-dimensional shape by an aspherical surface measuring machine, obtaining a shape error from the design values, and excluding a setting error with respect to the measuring machine from the shape error (Patent Document 2).
[Patent Document 1] Japanese Patent Application Laid-open No. 2003-117925
[Patent Document 2] Japanese Patent Application Laid-open No. 8-216272